Problem: Is ${551012}$ divisible by $3$ ?
Answer: A number is divisible by $3$ if the sum of its digits is divisible by $3$ . [ Why? First, we can break the number up by place value: $ \begin{eqnarray} {551012}= &&{5}\cdot100000+ \\&&{5}\cdot10000+ \\&&{1}\cdot1000+ \\&&{0}\cdot100+ \\&&{1}\cdot10+ \\&&{2}\cdot1 \end{eqnarray} $ Next, we can rewrite each of the place values as $1$ plus a bunch of $9$ s: $ \begin{eqnarray} {551012}= &&{5}(99999+1)+ \\&&{5}(9999+1)+ \\&&{1}(999+1)+ \\&&{0}(99+1)+ \\&&{1}(9+1)+ \\&&{2} \end{eqnarray} $ Now if we distribute and rearrange, we get this: $ \begin{eqnarray} {551012}= &&\gray{5\cdot99999}+ \\&&\gray{5\cdot9999}+ \\&&\gray{1\cdot999}+ \\&&\gray{0\cdot99}+ \\&&\gray{1\cdot9}+ \\&& {5}+{5}+{1}+{0}+{1}+{2} \end{eqnarray} $ Any number consisting only of $9$ s is a multiple of $3$ , so the first five terms must all be multiples of $3$ That means that to figure out whether the original number is divisible by $3 $ , all we need to do is add up the digits and see if the sum is divisible by $3$ . In other words, ${551012}$ is divisible by $3$ if ${ 5}+{5}+{1}+{0}+{1}+{2}$ is divisible by $3$ Add the digits of ${551012}$ $ {5}+{5}+{1}+{0}+{1}+{2} = {14} $ If ${14}$ is divisible by $3$ , then ${551012}$ must also be divisible by $3$ ${14}$ is not divisible by $3$, therefore ${551012}$ must not be divisible by $3$.